This invention relates to a method of and an apparatus for processing sampled analog electrical signals.
For processing analog signals using circuitry which can be easily integrated, the switched capacitor technique has been used. Since it is difficult to integrate large valued resistors because of the area occupied and further the correlation between resistor and capacitor values produced by integration is not good, causing the resultant time constants to be poorly defined, the technique of using the equivalent resistance of a capacitor which is switched into and out of circuit to cause processing to take place by means of manipulation of charge packets has been used for analog signal processing where integration has been required.
Although they are widely used, switched capacitor circuits have certain disadvantages. It is necessary to produce capacitors which are linear, that is their capacitance should not change significantly with the signal level. This has been achieved in CMOS integrated circuits by providing two polysilicon layers for the plates of the capacitors. However, standard CMOS processes used for the integration of digital circuits do not employ a double polysilicon layer . Consequently, switched capacitor circuits which are formed on the same chip as digital circuits require additional processing steps. In switched capacitor circuits the double polysilicon layer switched capacitors, together with the required compensation capacitors for the operational amplifiers, can account for a significant proportion of the total silicon area. This tends to produce relatively large chips. Furthermore in most switched capacitor systems, each amplifier and switch must be individually designed and optimized for its particular environment in order for the circuit to perform adequately.